Data in solid state storage devices, such as NAND (Not AND) storage devices, may decay over time when kept in an open state. Solid state storage devices typically write data to the storage medium as a storage unit (e.g., the amount of data that is erased in a single garbage collection cycle). In the case of sparsely and/or sporadically written user data, a storage unit may be opened by the controller and some data may be written to the storage unit, but the user may not provide enough data to fill the storage unit. In this case, all of the blocks in the storage unit must be kept in an open state until more data is received to fill the storage unit. A known limitation of NAND devices is that open blocks of data will degrade over time. If not monitored and the block closed or rewritten before the degradation becomes critical, the data written to open blocks could be lost. For this reason, NAND storage devices typically track open blocks and refresh them, as needed, to prevent data loss. This protects the data, but results in an extra Program/Erase cycle for the open block(s) and reduces the effective endurance of the solid state storage device.
A need therefore exists for improved techniques for reducing open block degradation in NAND flash memory devices.